The Internet Protocol (IP) multimedia subsystem (IMS) is a standardized architecture for providing multimedia services over networks supporting packet-based communications. IMS is configured to provide a centralized service control system across different packet network architectures, which are referred to as packet subsystems (PSs). As such, IMS can support multimedia services over different types of access networks. These access networks may support fixed or wireless communications, as long as there is a mechanism to support packet-based communications. IMS runs over the standard IP. IMS generally uses VoIP technology based on a third generation partnership project (3GPP) implementation of the Session Initiation Protocol (SIP). With IMS, services can be provided to subscribers regardless of their location, access technology, and terminal. A general overview of IMS operation follows.
User elements (UEs) are capable of communicating with each other via their respective access networks. For a given call or session between the user elements, the IMS architecture will provide the requisite signaling to establish and control the call or session. For conciseness and readability, calls or sessions are collectively referred to as calls. Further, the media delivered in the calls may be data, audio, video, or voice. The IMS architecture employs several call/session control functions (CSCFs), which are implemented as SIP servers or proxies. These CSCFs are used to process SIP signaling messages, which facilitate the signaling required for establishing and controlling the calls.
Session signaling may be provided in various networks, such as a visited network, a home network, and a called network. The visited network represents the network currently supporting a roaming user element of User A, who is the calling party originating a call. The home network is the home services network for the user element of User A, and the called network is a visited or home network for the user element of User B, who is the called party that is terminating the call.
Each of these networks may include CSCFs. The CSCFs are implemented having three primary functions: a proxy CSCF (P-CSCF), an interrogating CSCF (I-CSCF), and a serving CSCF (S-CSCF). The P-CSCF is a SIP proxy that is generally the first point of contact for a user element, and can be located in a visited network or home network. The P-CSCF in the visited network is associated with the user element of User A. User A's user element may be assigned to the P-CSCF in the visited network during registration. The P-CSCF in the visited network is in the signaling path of certain, if not all, signaling messages for the call, and will authenticate the user and establish a security association with the User A's user element. The P-CSCF in the visited network may also compress and decompress SIP messages in an effort to reduce signaling overhead or increase response times over slow radio links. Further, the P-CSCF may map the user ID associated with the user element to an appropriate I-CSCF.
To initiate the call with User B's user element, User A's user element will send to the P-CSCF in the visited network a session initiation message identifying User B's user element as the called party. The P-CSCF in the visited network will route the call to the I-CSCF in the home network. The I-CSCF is also a SIP proxy and is generally located at the edge of an administrative domain, which is the home network in this example. The IP address of the I-CSCF is published in the domain name service of the administrative domain, such that the P-CSCF in the visited network, as well as any other entities, can locate the I-CSCF and use it as a point of entry for all signaling messages for the administrative domain.
Upon receipt of the session initiation message from the P-CSCF in the visited network, the I-CSCF may access a home subscriber server (HSS) to identify the S-CSCF to use for session control. The HSS is essentially a master database that supports various network entities that are involved in establishing and controlling calls. The HSS contains user profiles and other related subscription information, assists in authentication and authorization of a user, and can provide information about the physical location of a user by keeping track of the location of the user element.
The S-CSCF is generally the central signaling node in the IMS architecture for the user element of User A. Upon receipt of the session initiation message from the I-CSCF in the home network, the S-CSCF in the home network will may access the HSS to determine the location of User B's user element and identify the S-CSCF in the called network to use for session control. The S-CSCF in the home network will then route the session initiation message to the S-CSCF in the called network, which is serving User B's user element.
Upon receipt of the session initiation message from the S-CSCF in the home network, the S-CSCF in the called network will route the call to the P-CSCF, which is serving User B's user element in the called network. The P-CSCF in the called network will then route the session initiation message to the user element.
The S-CSCFs used for the routing the session initiation message remain in the signaling path for subsequent signaling messages for the call supported between the user elements, and can inspect signaling messages traveling in either direction. Similarly, the P-CSCFs and I-CSCFs, which are invoked during initial routing, may remain in the signaling path and handle signaling messages exchanged between the user elements during the call. The bearer path is provided over a transport plane and session control is provided in the control plane, which is generally comprised of the CSCFs.
Based on inspecting the signaling messages, the S-CSCFs can determine if and when to invoke multimedia services for the user elements or associated calls. The multimedia services are provided in a service plane, which is generally made up of numerous application servers, which are capable of providing one or more multimedia services. To provide a multimedia service, the S-CSCF will identify the appropriate multimedia service and forward signaling messages to the application server chosen to provide the multimedia service. The application server will provide the multimedia service by effectively processing the signaling message and returning the processed signaling message back to the S-CSCF, if necessary, which will forward the signaling message in a desired fashion. The application server providing a selected multimedia service will also reside in the signaling path. With IMS, call control and service presentation is virtually limitless.
Although IMS provides tremendous flexibility for controlling calls and facilitating multimedia services within packet subsystems (PSs), circuit-switched subsystems (CSs) continue to support a vast majority of voice-based communications. In light of the coverage of CSs and the benefits of IMS, efforts are underway to interwork CSs and PSs, as well as control calls and provide associated multimedia services associated with the calls using IMS. Current efforts generally employ a PS for calls, even if a CS is available. Given the current state of PSs, CSs often provide better quality of service for voice calls due to network capability, loads, or environment conditions. Thus, IMS controlled calls that would benefit from being provided at least in part by an available CS are still supported by the PS. As such, there is a need to for an effective and efficient technique to establish calls through the CS while controlling the calls and providing associated services using IMS. There is a further need to maintain such control across domain transfers between CS and PS domains.